As is known in the art, in wireless communications, multipath is a propagation phenomenon that results in wireless communications signals reaching a receiver by more than a single path. There are many causes of multipath including atmospheric effects and obstructions from terrestrial objects such as mountains and buildings. The effects of multipath include cross-interference of the multipath signals and phase shifting caused by the varying geographic lengths each electromagnetic signal must travel.
Multipath can be modeled using impulse response methods for linear systems. For example, a wireless communications transceiver will transmit an original signal, b, subject to multipath. More than one version of the original signal, which may be provided as an impulse, will arrive at different times at a wireless communications receiver. The versions of the signal will often be attenuated. The received signal s at the receiver at time t can be represented using the following equation:s(t)=Σ(n=0,N-1)b(t−τn)gn(t−τn)ejθn(t-τn) 
In this equation, N is the number of received impulses, which can be very large due to the number of paths of the signal. τn represents the time delay experienced by the nth path. gnejθn is the complex amplitude and phase response of each path.
In digital communications, multipath can cause inter-symbol interference (ISI) errors and affect the quality of communications. Hardware and software techniques can be employed to correct ISI, including equalizers, orthogonal frequency division modulation, and rake receivers.
As is also known, a rake receiver is used to counteract the effects of multipath using multiple sub-receivers built into a receiver. The sub-receivers are often referred to as fingers because they are analogous to the function of the fingers on a garden rake, i.e., each finger collects communications bits similar to how each finger of a garden rake collects different leaves.
In a rake receiver, each finger tunes to a different multipath component of the signal. Each finger decodes the signal and outputs it to a combiner. The combiner uses the different transmission characteristics of the outputs to produce a signal, typically having a higher signal-to-noise ratio than the separate signals.
Rake receivers are common in Code Division Multiple Access (CDMA) and Wide-CDMA (W-CDMA) used by various radio communications technologies and well-known in the art. These wireless communications technologies can experience undesired frequency shifts in radio communications due to the Doppler Effect. The Doppler Effect is an observed shift in the frequency of traveling waves caused by relative movements between a wave transmitter and a wave receiver. In wireless radio, the shifts in frequency depend on whether the wave transmitter, for example, a mobile radio unit, is moving toward or away from the wave receiver, for example, a radio tower. Doppler Effect produces a received frequency corresponding to the transmitted frequency offset by the shift in frequency.
The Doppler Effect can be represented as follows:fd=fv/c fd is the shift in frequency, v is the velocity of the transmitter relative to the receiver, and c is the speed of an electromagnetic wave. The observed frequency f′ will equal the transmitted frequency f plus (or minus) fd as follows:f′=f±fd or f′=f±fv/c 
Existing rake receivers can adapt to relatively slow changing phase and amplitude variations within a multipath environment. However, existing rake receivers cannot adapt fast enough to counteract significant Doppler shift experienced during wireless radio communications from high-speed transmitters. In such instances, the Doppler Effect can degrade performance of a Direct-Sequence Spread Spectrum (DSSS) waveform commonly used in CDMA communications technologies. DSSS is a modulation technique well known in the art.
DSSS phase-modulates an information signal pseudo-randomly with a continuous string of pseudo-noise (PN) code symbols known as chips. The chips have a shorter duration than the information bits. In this way, DSSS modulates the information bits by a sequence of much faster chips, producing a chip rate that is much higher than the information bit rate. A receiver can receive the transmitted signal and use the same string of PN code symbols to reconstruct the information signal.
It would, therefore, be desirable to provide a rake receiver and a method of operating a rake receiver to counteract the Doppler Effect in wireless radio communications regardless of the amount of any Doppler shift caused by movement of receivers and transmitters. It would also be desirable to provide a rake receiver and method of operation which allows the rake receiver to adapt to relatively large Doppler shifts in a multipath environment.